This invention is concerned with improvements in and relating to apparatus and methods for materials investigations. The invention is particularly, but not exclusively, concerned with investigating gamma ray emissions from materials. The invention is still more particularly, but not exclusively, concerned with correction techniques including the provision of more suitable transmission sources and an improved correction technique.
In a variety of situations it is necessary to investigate emissions from radioactive sources in or on materials to form a basis for a variety of subsequent decisions, actions or further considerations. The investigations of the sample may relate directly to the emission, for instance the emission source, or indirectly, for instance the consideration of associated non-emitters or emitters which are not directly measurable. The emissions of interest are in particular gamma ray emissions, but other emission forms may be considered additionally or alternatively.
Emission investigation is particularly important in waste evaluation cases. For a given waste sample it is desirable to be able to determine a variety of unknowns. The unknowns may include, but are not limited to, one or more of the level, type, constituents, nature and distribution of the emissions, emission sources, associated materials or associated factors.
As well as simply using the results obtained from the detectors, emission count rates for the radioactive materials in the sample under consideration, it is known to investigate to an extent the effects of the all the material in a sample, through their effect on the transmission of external emissions from an applied source through the sample to the detectors.
The applicant has determined that such simple sources are not suitable for transmission based investigation and correction at the full variety of energies and with the full variety of materials. As a consequence the effectiveness of the correction technique based on these investigations is impaired.
The present invention seeks to provide a more widely accurate transmission based correction, which is also simple and easy to use across a full spread of situations which are encountered in practice.
In addition the existing transmission based correction techniques only go part way to account for the variables which affect the detected counts for the sources of the sample itself and as a result the correction technique is not fully effective.
The present invention has amongst its aims the provision of a more accurate and thorough correction technique based on transmission correction and the manner in which it is employed.
According to a first aspect of the invention we provide a method of investigating radioactive sources in a sample, the method comprising detecting a portion of the emissions arising from the sample, and further comprising the provision of a radioactive generator, passing at least a portion of the emissions of the generator into the sample, detecting at least a portion of the emissions from the generator leaving the sample, the radioactive generator emissions being of at least a plurality of emission energies and at least two of those energies being detected.
Preferably the method further provides that the detected portion of the source emissions relate to a detected level for the sources in a sample, the detected level being corrected according to a correction method to give a corrected level for the sources in a sample, the process being repeated for one or more other samples.
Preferably the correction method employs measured transmission coefficients in determining the correction. The measured transmission coefficients, for one or more of the energies, most preferably all, may be provided according to the equation:       Trans.  Coeff.    =      R          R      o      
where R is the rate of detected photons with the sample in place, Ro is the rate of photons which would be detected without the sample in place.
Preferably the density determined is used as a factor in the correction method. The density used in the correction method may be an averaged density from the determinations or a weighted average density from the determinations.
For correction of source emission energies corresponding to a generator energy preferably the measurement based correction factor is used. For correction of source emission energies not corresponding to a generator energy preferably the correction factor is based on the extrapolation of the correction factors based on the measurements.
The generator is preferably a single isotope. Preferably the emission energies extend across a substantial portion of the range of energies emitted from the sample. A substantial portion may be 50%, preferably 75%, more preferably 90% and ideally 100% of the sample energies range. The generator most preferably of all emits energies encompassing the range of energies emitted by the sample. 152Eu is a particularly preferred generator. Preferably at least 5 energies from the source are detected and used, more preferably at least 8 energies are detected and used.
Preferably the portion of generator emissions detected have passed through the sample. Preferably the generator is provided on the opposing side of the sample to the detectors, most preferably in direct opposition.
One or more of the detectors for the sources may be used for detecting the generator emissions and/or vice-versa.
According to a second aspect of the invention we provide apparatus for investigating radioactive sources in a sample, the apparatus comprising:
one or more detectors for emissions from the sources, the detectors generating signals indicative of the emissions detected;
an investigating location into which the sample is introduced;
signal processing means for relating the detector signals to one or more characteristics of the sources;
a radioactive emission generator separate from the sample; and
one or more detectors for emissions from the radioactive generator leaving the sample;
wherein the radioactive generator emissions are of at least a plurality of energies and a least two of the plurality of energies are detected.
The source detectors and the generator detectors may be one and the same in the case of one or more or all of the detectors.
The second aspect of the invention may include any of the features, options and possibilities set out on the first aspect of the invention, including apparatus suitable for the implementation of the method steps detailed therein.
According to a third aspect of the invention we provide a method of investigating radioactive sources in a sample, the method comprising detecting a portion of the emissions arising from the sample at an energy, the detected portion relating to a detected level, the detected level being corrected according to a correction method to give a corrected level, at that energy, the correction method including:
the provision of an emission generator, passing at least a portion of the emissions of the generator into the sample, detecting at least a portion of the emissions from the generator leaving the sample, and determining a value for a first relationship between the two portions;
calculating a value for a relationship of equivalent type to the first, the calculation being based on functions of an element""s absorption of emissions and the amount of that element potentially encountered by emissions, for one or more elements;
adjusting one or more variables/functions in the calculated relationship to reduce the difference between the value of the determined relationship and the value of the calculated relationship for the sample at a plurality of the energies of emissions from the generator; and
obtaining the values of the calculated relationship functions from the reduction and calculating the calculated relationship from those factors at the sample source emission energy requiring correction and correcting the detected level using those values.
Preferably the generator emissions are of at least two emission energies, ideally multi-energy emissions are provided, and/or with at least two of those energies being detected.
The generator is preferably a single isotope. Preferably the emission energies extend across a substantial portion of the range of energies emitted from the sample. A substantial portion may be 50%, preferably 75%, more preferably 90% and ideally 100% of the sample energies range. The generator most preferably of all, emits energies encompassing the range of energies emitted by the sample. Eu152 is a particularly preferred generator.
Preferably the portion of generator emissions detected have passed through the sample. Preferably the generator is provided on the opposing side of the sample to the detectors, most preferably in direct opposition.
One or more of the detectors for the sources may be used for detecting the generator emissions and/or vice-versa.
Preferably the first relationship employs measured transmission coefficients, and ideally comprises measured transmission coefficients. The measured transmission coefficients, for one or more of the energies, most preferably all, may be provided according to the equation:       Trans.  Coeff.    =      R          R      o      
where R is the rate of detected photons with the sample in place, Ro is the rate of photons which would be detected without the sample in place.
Preferably the calculated relationship is based on functions addressing one or more of the density, emission path length in the sample and sample absorption of emissions. Preferably the calculated relationship is based on functions addressing one or both of the effect of the material forming the sample over the emission path length in the sample and the sample absorption of emissions.
The calculated relationship is preferably based on the equation:
Ti=exp(xe2x88x92xcexa3qjxc2x7xcexci,j) 
where Ti is the transmission coefficient at the energy i under consideration; qj is the effective material thickness or the effect of the specified material forming the sample over the specified emission path length through the sample, for element j; xcexci,j is the mass absorption coefficient for elements j at energy i. The sum including all of the specified elements, j, included in the method.
Preferably the calculated relationship includes contributions from two or more elements, and most preferably three or more. The elements may be elements in the sample, likely to be in the sample, unknown or not in the sample.
Most preferably the elements include at least one low atomic mass element, preferably less than 10 and ideally less than 5. Preferably the elements include at least one high atomic mass element, preferably greater than 40 and ideally greater than 50. Preferably the elements include at least one intermediate atomic mass element, preferably between 10 and 50 and ideally between 10 and 40. Ideally at least one element from each category is provided.
Preferably the adjusting of the variables/functions/factors varies one or two of the factors only. Preferably only the effective material thickness or the effect of the specified material forming the sample over the specified emission path length through the sample are varied, particularly where one factor only is varied.
The reduction in the differences between the first relationship value and calculated value may be undertaken so as to reduce the overall difference between all of the first relationship and calculated relationship values involved. The reduction may be intended to minimise the difference. The reduction process may be a minimisation of the sum of the residuals.
Preferably the method is repeated for one or more other samples, the one or more other samples may, with the sample, form a body of material under investigation.
For the correction of source emission energies corresponding to a generator emission energy, preferably the measurement based correction factor alone, i.e. the first relationship, is used.
According to a fourth aspect of the invention we provide apparatus for investigating radioactive sources in a sample, the apparatus comprising:
one or more detectors for emissions from the sources, the detectors generating signals indicative of the emissions detected;
an investigating location into which the sample is introduced;
signal processing means for relating the detector signals to a detected level for the sources;
processing means for correcting the detected level for the sources, according to a correction method, to give a corrected level;
a radioactive emission generator separate from the sample;
one or more detectors for emissions from the generator which leave the sample;
processing means for determining a first relationship, based on the portion of generator emissions entering the sample and the portion of generator emissions leaving the sample;
processing means for calculating a value for a relationship of equivalent type to the first, the calculation being based on functions of an element""s absorption of emissions and the amount of that element potentially encountered by emissions, for one or more elements;
processing means for adjusting one or more variables in the calculated relationship to reduce the difference between the value of the determined relationship and the value of the calculated relationship for the sample at a plurality of the energies of emissions from the generator; and
calculating means for obtaining the values of the calculated relationship functions from the reduction and calculating the calculated relationship from those factors at the sample source emission energy requiring correction and correcting the detected level using that value.
The processing means may be separate from one another or a common unit in one or more cases, including the calculating means.
The fourth aspect of the invention may include any of the features, options and possibilities set out on the third aspect of the invention, including apparatus suitable for the implementation of the method steps detailed therein.
The first and/or second and/or third and/or fourth aspects of the invention may further include any of the features, options, possibilities and steps set out below.
The sources may be singular or plural in disposition and/or type. The sources may be one or more isotopes of one or more elements. The sources may be alpha and/or beta and/or gamma emitters, both are preferably gamma emitters at least.
One or more sources of the same type and/or of different types may be present in the sample. The sources may be homogeneously distributed, or more usually, unevenly distributed. The size and/or shape and/or mass of a source may be different from the size and/or shape and/or mass of another source in the sample, be they of the same or different types.
The sources may be investigated by detecting one or more of their emitting energies. Thus a characteristic energy of an isotope may be detected.
The sources may be investigated directly, for instance they contribute directly to the detected level, and/or the sources may be investigated indirectly, for instance they do not contribute directly to the detected level but are associated with sources which do.
The samples may be gaseous and/or liquid and/or solid. The samples may contain one or more non-emitting or non-source materials. The materials may include one or more of metals, such as iron, steel, aluminum; wood; glass; plastics, such as polythene, PVC; liquids, such as water.
The sample may be the whole or part of a body of material. The body of material may be free standing, but is preferably contained in a container. The sample may be a part of a body of material, including the part of the container associated with that part of the body of material.
The sample may be a segment or slice through a body of material. Preferably the segment is taken horizontally through the body of material. Preferably the segment has the same thickness throughout the body of material.
Other segments of the body of material may be investigated in subsequent repeats of the method.
The container preferably entirely encloses the body of material. The container may be of metal or of concrete or a combination of such materials. Drums are a particularly preferred container, such as right cylindrical drums.
The containers may be of one or more standard sizes. The height and/or diameter of the containers may be standard.
Preferably the containers introduced to the investigating location one at a time. The containers may be introduced by conveying along a surface, preferably a horizontal surface. The surface may include or be formed of a plurality of rollers. Preferably the container is removed from the investigating location in a manner equivalent to its introduction.
The investigating location is preferably provided in proximity to the emission detector or detectors. The investigating location may be provided in proximity to one or more radioactive sources. The sources are preferably intended to transmit radiation through the sample. Ideally the investigating location is provided between the detector(s) and the transmission source(s).
The sample, preferably the container for it, may be rotated at the investigating location. Preferably the rotation presents different portions of the sample in proximity to the detector(s) and/or transmission source(s). The rotation may be continuous or stepped. The rotation may be provided at between 5 and 25 rpm.
Preferably the sample and/or body of material and/or container are weighed, most preferably at the investigating location, for instance by the turntable used to rotate it.
The sample, preferably the container for it, may be raised and/or lowered at the investigating location. Preferably the rasing and/or lowering presents different portions of the sample to the detector(s) and/or transmission source(s). The raising and/or lower may be continuous or stepped. Preferably investigations are performed as the sample is lower and raised.
The sample may be rotated and/or lower and/or raised.
A single detector may be used. Preferably a plurality of detectors, for instance three, may be used.
The detectors may be of the high purity germanium type.
Preferably the detectors are collimated to restrict their field of view to the body of material of which the sample is the whole or a portion thereof. Where the sample is less than the whole of the body of material, preferably the detectors are collimated to restrict their field of view to the sample only. The sample is preferably a slice or segment of the whole. The segments may be of the same thickness.
Preferably the detected level is obtained from a passive counting stage. Preferably the transmission based investigations are performed before and/or after the passive count stage.
Preferably the transmission source(s) is provided in opposition to the detector(s). Preferably the same number of transmission sources are provided as there are detectors. It is particularly preferred that the transmission source be provided according to nature of the transmission source detailed in the first and/or second aspect of this invention.
One or more surface dosimeters may be provided. Preferably the surface dosimeters are configured to investigate gamma emitting sources. Alpha and/or beta emissions may alternatively or additionally considered.
Preferably the correction method is used in correcting the detected level to the corrected level, for instance by established subsequent techniques, such as those set out in the Los Alamos primer, 2nd Edition, March 1991, ISBN0-16-032724-5.